Simulation and characterization methods of complex fracture network expansion in tight sandstone

Abstract

The present invention provides a method for simulating and characterizing the expansion of complex fracture networks in tight sandstones, including: analyzing the mineral composition of tight sandstone cores; testing the rock mechanical parameters of the tight sandstone cores; testing the magnitude and orientation of the maximum horizontal principal stress in the tight sandstone cores; scanning the microcrack distribution patterns in the tight sandstone cores using CT technology; preparing cement-coated tight sandstone core samples with rock mechanical properties and microcrack distribution patterns similar to those of natural tight sandstone cores; establishing laboratory test parameters for fracturing complex fracture networks in tight sandstones; exploring the expansion patterns of complex fracture networks in tight sandstones under different microcrack distribution patterns; and using CT technology to construct a three-dimensional data volume from the cores before and after the experiment to characterize the complex fracture network. This method quantitatively analyzes the influence of microcracks and mineral composition on the expansion of complex fracture networks in tight sandstones, providing an experimental basis for optimizing on-site fracturing schemes for tight sandstones.

Description

Technical Field

[0001] The present invention relates to the technical field of complex fracture network transformation in tight sandstone volume fracturing, and in particular to a method for simulating and characterizing the expansion of a complex fracture network in tight sandstone. Background Art

[0002] Tight sandstone gas reservoirs are currently a hot energy source for exploitation, but their permeability is extremely low, and they require hydraulic fracturing to form a complex fracture network to obtain economical oil and gas flows.

[0003] Tight sandstone reservoirs have extremely low permeability and porosity, and low formation pressure. Influenced by formation temperature, pressure, and source rock evolution, tight sandstones exhibit significant heterogeneity, with significant variations in mineral composition and content, formation brittleness, and the development of microcracks. During the formation of complex fracture networks in tight sandstones, structural weaknesses such as mineral composition and content, microcracks, and microfractures significantly influence the expansion of these complex fracture networks, leading to bifurcation and diversion within microcracks and mineral compositions. Factors such as microcrack parameters (microcrack length, density, aperture, and orientation), the magnitude and variation of in-situ stresses, and fracturing fluid application parameters (injection rate, displacement, viscosity, and temperature) all influence the expansion of complex fracture networks in tight sandstones through volumetric fracturing. Therefore, optimizing fracturing process parameters and analyzing the influence of factors such as microcracks on the expansion of complex fractures in tight sandstones are of great significance.

[0004] Large-scale indoor true triaxial physical simulations can effectively simulate the complex fracture propagation patterns under the influence of various factors. However, current fracturing experiments primarily use artificial specimens with prefabricated microfracture weak planes or outcrop cores. These experiments fail to accurately reflect the state of deep, dense sandstone cores and simulate the propagation patterns of complex fracture networks.

[0005] Conventional experimental methods are difficult to accurately evaluate the growth characteristics of hydraulically fractured tight sandstone networks at different geological levels. Therefore, an effective fracture network evaluation method for complex fracture networks in tight sandstone formations is urgently needed. This method can accurately describe the microcrack parameters of tight sandstone and quantitatively analyze the influence of microcracks on the growth of hydraulic fracture networks, thereby providing basic data and theoretical reference for field fracturing operations.

[0006] The Chinese patent application, application number CN201510752304.8, involves a method for predicting complex network fractures in tight sandstone reservoirs. The method involves the following steps: establishing a structural geological model and a fracture development model; testing the magnitude and direction of ancient and present-day geostress; conducting complete rock mechanics parameter experiments; testing rock mechanics parameters and fracture stress sensitivity; establishing a multi-stage composite fracture criterion for fractured rocks; conducting physical testing of rock deformation to obtain peak strength; establishing a model for the relationship between uniaxial stress-strain and fracture density; establishing a model for the relationship between triaxial stress-strain and fracture density and occurrence; establishing a model for the relationship between uniaxial stress-strain and fracture density; calculating and simulating fracture parameters under current conditions; and verifying the reliability of the quantitative fracture prediction results.

[0007] Chinese patent application number CN201510867900.0 describes a method for evaluating the spatial effectiveness of complex fracture networks in tight sandstone reservoirs. This method involves establishing a discrete fracture network geological model, statistically analyzing the fracture formation stages, the sources of fracture fillings, the dominant fracture filling directions and formations, analyzing the primary geological factors controlling fracture filling, establishing a relationship between fracture permeability and present-day principal stress, and evaluating the spatial effectiveness of the fracture network system.

[0008] The Chinese patent application with application number CN201910732445.1 involves a modeling method for natural fractures in tight sandstone reservoirs, which includes the following steps: 1) identifying the internal structure of single well fractures and fault fracture zones; 2) based on sedimentary phase control and fracture seismic interpretation, a modeling method combining deterministic and multiple random algorithms is adopted to establish a matrix geological model under lithofacies-tectonic coupling; 3) under the constraints of the lithofacies-tectonic coupled matrix geological model, a three-dimensional heterogeneous rock mechanics model is established by adopting a sequential indicator simulation combined with a trend modeling method of a vertical probability body; 4) finite element numerical simulation is performed on the three-dimensional heterogeneous rock mechanics model to obtain a three-dimensional spatial stress field model; 5) by establishing the relationship between the stress field and fracture parameters, a fracture model is established using a discrete fracture network modeling method.

[0009] The above existing technologies are significantly different from the present invention and fail to solve the technical problem we want to solve. Therefore, we have invented a new method for simulating and characterizing the expansion of complex fracture networks in dense sandstone. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for simulating and characterizing the expansion of complex fracture networks in dense sandstones by using CT technology to accurately describe the distribution of microcracks and mineral components in dense sandstones and to quantitatively analyze the influence of microcracks and mineral components in dense sandstones on the expansion of complex fracture networks.

[0011] The object of the present invention can be achieved by the following technical measures: a method for simulating and characterizing the expansion of a complex fracture network in tight sandstone, which includes:

[0012] Step 1, analyzing the mineral composition of the tight sandstone core;

[0013] Step 2: Testing rock mechanical parameters of tight sandstone core;

[0014] Step 3: testing the maximum horizontal principal stress and orientation of the tight sandstone core;

[0015] Step 4: Use CT technology to scan the distribution pattern of microcracks in the dense sandstone core;

[0016] Step 5: preparing a cement-coated dense sandstone core sample having similar rock mechanical properties and microcrack distribution patterns to those of natural dense sandstone cores;

[0017] Step 6: Develop test parameters for fracturing complex fracture networks in tight sandstone in the laboratory based on the similarity principle.

[0018] Step 7: Conduct a large-scale true triaxial complex fracture network physical simulation experiment to explore the expansion law of complex fracture networks in tight sandstone under different microcrack distributions;

[0019] Step 8: Use CT technology to create a three-dimensional data volume of the core before and after the experiment to represent the complex fracture network.

[0020] The purpose of the present invention can also be achieved by the following technical measures:

[0021] In step 1, a mineral component analyzer is used to analyze the mineral composition and content of the dense sandstone, including quartz, calcite, and clay, and the dense sandstone brittleness index is calculated to determine the complexity of the fracture network.

[0022] In step 2, the rock mechanics parameters of the dense sandstone core, such as shear strength, elastic modulus, and Poisson's ratio, are tested; and the rock mechanics parameters, such as peak strength, elastic modulus, and Poisson's ratio, of the dense sandstone core are tested using a triaxial testing device for rock mechanics parameters.

[0023] In step 3, the maximum and minimum horizontal principal stresses of the core are tested using differential strain experiments, the original orientation of the tight sandstone core is determined using paleomagnetic tests, and a tight sandstone core coordinate system is established; the in-situ stress orientation is determined based on the tight sandstone core logging data.

[0024] In step 3, the logging data includes sonic, caliper, and resistivity logging.

[0025] In step 4, CT technology is used to scan the distribution of microcracks in the dense sandstone core before the experiment to obtain its distribution pattern, including microcrack density, length, and openness. A digital image space coordinate system is established to obtain a three-dimensional data volume to calculate the azimuth and inclination of the microcracks. The image is processed to make it clearer and the microcracks more obvious. Then, weighted average interpolation is performed on the image, and a standard layer is selected and set as the initial image.

[0026] In step 4, the microcrack parameters of tight sandstone are obtained by taking the center point of the rock sample as the coordinate origin (0,0,0) of the spatial coordinate system XYZ; the three-dimensional data volume of the core is obtained by CT scanning, and the coordinates of the first and last two pixel points of the microcrack (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 ) to determine the length and distribution azimuth angle α of the microcracks; draw a frequency histogram of the azimuth angle α of the microcracks in the analysis section and the deflection angle Y in the direction of the maximum horizontal principal stress to determine the range of variation of the microcrack parameters.

[0027] In step 4, the deflection angle Y between the microcracks in the tight sandstone and the maximum horizontal stress orientation is determined based on the calibrated maximum horizontal stress orientation of the tight sandstone.

[0028] In step 5, the proportions of water, quartz sand, lime, and clay are adjusted, the content of the fluid loss reducer additive is determined, and a cement-coated dense sandstone core sample having rock mechanical properties and microcrack distribution similar to those of a natural dense sandstone core is prepared.

[0029] In step 5, the proportions of water, quartz sand, lime and additives are adjusted respectively to prepare the rock mechanical parameters of the cement sample similar to those of the dense sandstone rock mechanical parameters; a through hole with a diameter of 25 mm is drilled in the center of the natural dense sandstone core; the prepared cement slurry is placed in regular hexahedral steel molds of different sizes, and then the natural dense sandstone core is placed in the center and the direction is adjusted so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and marked; then the steel wellbore is cast in the cement mortar together, and the wellbore perforations are filled with urea in advance; it is cured at a constant temperature of 20°C for 28 days, and then the surface of the large-size sample is treated to make its parallelism and verticality meet the requirements, and then the test can be carried out.

[0030] In step 6, the similarity principle is used to prepare the laboratory test parameters for the fracturing construction of complex fracture networks in tight sandstone. Based on the rock mechanical parameters, ground stress, brittleness index and microcrack distribution characteristics of tight sandstone, a large-scale true triaxial physical simulation test fracturing construction plan is designed, including fracturing fluid viscosity, fracturing fluid displacement, fracturing fluid type, and fracturing fluid injection volume. Tracer materials are added to the fracturing fluid to facilitate the analysis of the expansion law of complex fracture networks in tight sandstone.

[0031] In step 7, a large-scale true triaxial physical simulation experiment of complex fracture network in tight sandstone is carried out to test the expansion law of complex fracture network in tight sandstone under different construction parameters, different mineral components and contents, and different microcrack distributions; using the prepared large-scale tight sandstone rock samples, a large-scale true triaxial physical simulation experiment is carried out according to the set experimental plan.

[0032] In step 8, CT technology is used to scan and test large-scale rock samples and dense sandstone cores after the experiment, establish a three-dimensional data body, and analyze the changing characteristics of microcracks in the dense sandstone cores. Different large-scale physical model experimental schemes are used to determine the influence of geological conditions, construction process parameters, mineral composition, and microcrack distribution characteristics on the expansion of complex fracture networks in dense sandstone.

[0033] Step 8 specifically includes:

[0034] After testing, large-scale rock samples were taken and placed under industrial CT to analyze the distribution patterns of complex fractures in the large-scale rock samples. Natural dense sandstone cores were taken from the large-scale rock samples and scanned using CT to obtain three-dimensional natural fracture and fracture propagation data in the dense sandstone cores. The effects of dense sandstone microcracks and rock mineral composition on the propagation characteristics of complex fracture networks were analyzed.

[0035] The data volume is set according to the calibration position, and the change pattern of microcrack length, density and opening width before and after fracturing is obtained by analysis; the orientation of the complex fracture network is determined, and the relationship between the complex fracture network and the microcrack distribution is statistically analyzed, and the influence of microcrack parameters including length, density, opening degree and orientation on the expansion of the complex fracture network is quantitatively evaluated; a relationship chart of complex fracture network expansion parameters including length, orientation and microcrack parameters is established to quantitatively characterize the expansion pattern of the complex fracture network.

[0036] The method for simulating and characterizing the expansion of a complex fracture network in tight sandstone in the present invention comprises the following steps: analyzing the mineral composition of the tight sandstone; testing rock mechanical parameters such as shear strength, elastic modulus, and Poisson's ratio of the tight sandstone; calibrating the maximum horizontal ground stress, minimum horizontal principal stress, and orientation of the tight sandstone core; using CT to scan the tight sandstone core to obtain the distribution pattern of microcracks in the tight sandstone core; adjusting the water-lime-sand-clay content ratio to prepare a large-scale rock sample of the cement-coated tight sandstone core with similar rock mechanical properties; using the similarity principle to set experimental parameters for complex fracture network fracturing construction; conducting a large-scale true triaxial hydraulic fracturing physical simulation experiment on the complex fracture network in tight sandstone; using CT to scan the large-scale rock sample and the tight sandstone core after the experiment to obtain a fracture network expansion data volume; determining the influence of microcracks and mineral composition of the tight sandstone sample on the expansion of the complex fracture network, and digitally characterizing the complex fracture network. The present invention uses CT technology to accurately describe the distribution of microcracks and mineral components in dense sandstone, quantitatively analyzes the influence of microcracks and mineral components in dense sandstone on the expansion of complex fracture networks, and provides an experimental basis for optimizing on-site dense sandstone fracturing schemes.

[0037] The technical problem to be solved by the present invention is that during on-site downhole fracturing, the mechanical properties, bedding cementation degree and microcrack characteristics of dense sandstone layers in different work areas / layers are significantly different. Conventional experimental methods are difficult to analyze the expansion characteristics of the hydraulic fracture network of dense sandstone in different geological layers downhole, and to evaluate the influence of the ground stress orientation and microcrack orientation on the hydraulic complex fracture network.

[0038] To solve the above problems, the present invention conducts hydraulic fracturing physical simulation tests using rock samples from different layers in the well, adopts CT images to accurately describe the bedding of tight sandstone reservoirs and the degree of opening of microcracks, and quantitatively analyzes the influence of reservoir bedding and microcracks on the expansion of hydraulic fracture networks, providing an experimental basis for optimizing on-site tight sandstone fracturing schemes.

[0039] Compared with the existing technology, the beneficial effects are:

[0040] (1) Through the analysis of the mineral components of dense sandstone, the mineral composition and content of dense sandstone were clarified; at the same time, the rock mechanical parameters such as elastic modulus, Poisson's ratio and shear strength of dense sandstone were obtained through testing and analysis. By adjusting the water-lime-sand-additive content, a large-scale artificial rock sample of natural rock core coated with cement mortar with rock mechanical properties similar to those of natural dense sandstone was prepared, and the physical simulation results were more accurate.

[0041] (2) Based on the principle of similarity, a large-scale true triaxial physical simulation experiment scheme for complex fracture networks in tight sandstone was determined. At the same time, during the experiment, the temperature, pressure, pore fluid and ground stress conditions similar to those of the tight sandstone reservoir were simulated, and the simulation results were closer to the actual working conditions.

[0042] (3) Using CT scanning technology, the distribution pattern of microcracks in tight sandstone cores before and after the experiment and the expansion pattern of complex fracture networks were tested. A three-dimensional data body was established to represent the expansion morphology of complex fracture networks. The density, length, opening, and azimuth changes of microcracks in tight sandstone cores before and after fracturing, as well as the expansion pattern and azimuth angle of complex fracture networks, were analyzed. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a flow chart of a specific embodiment of the method for simulating and characterizing complex fracture network expansion in tight sandstone according to the present invention;

[0044] Figure 2 A three-dimensional data volume diagram of a dense rock core in a specific embodiment of the present invention;

[0045] Figure 3 This is a schematic diagram of a large-scale rock sample of dense sandstone in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0046] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0047] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations and / or combinations thereof.

[0048] The method for simulating and characterizing the expansion of complex fracture networks in dense sandstones of the present invention comprises the following steps:

[0049] (1) Analyze the mineral composition of tight sandstone cores;

[0050] The mineral composition and content of dense sandstone, including quartz, calcite, clay, etc., were analyzed using a mineral component analyzer to calculate the brittleness index of dense sandstone and determine the complexity of the fracture network.

[0051] (2) Test the rock mechanical parameters such as shear strength, elastic modulus, Poisson's ratio, etc. of the dense sandstone core; use the rock mechanical parameter triaxial testing equipment to test the rock mechanical parameters such as peak strength, elastic modulus, Poisson's ratio, etc. of the dense sandstone core.

[0052] (3) Measure the maximum and minimum horizontal principal stresses of the tight sandstone core and calibrate their orientations. Use differential strain tests to measure the maximum and minimum horizontal principal stresses of the core, use paleomagnetic tests to determine the original orientation of the tight sandstone core, and establish a tight sandstone core coordinate system. Determine the in-situ stress orientation based on well logging data from the tight sandstone core. Well logging data includes acoustic wave, caliper, and resistivity logging.

[0053] (4) Use CT technology to scan the distribution of microcracks in the dense sandstone core before the experiment to obtain its distribution pattern, including microcrack density, length, and opening, etc., establish a digital image space coordinate system, and obtain a three-dimensional data body to obtain the azimuth and inclination of the microcracks; process the image to make it clearer and the microcracks more obvious, and then perform weighted average interpolation on the image, and select a standard layer and set it as the initial image. The parameters of the microcracks in the dense sandstone are obtained, and the center point of the rock sample is used as the coordinate origin (0,0,0) of the spatial coordinate system XYZ; obtain the three-dimensional data body of the core through CT scanning, and the coordinates of the first and last two pixel points of the microcracks (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 ) to determine the length and distribution azimuth angle α of microcracks. A frequency histogram of the azimuth angle α and the deflection angle Y of the maximum horizontal principal stress direction within the analysis section was plotted to determine the range of variation of microcrack parameters. Based on the calibrated maximum horizontal in-situ stress azimuth of the tight sandstone, the deflection angle Y between the microcracks in the tight sandstone and the maximum horizontal in-situ stress azimuth was determined.

[0054] (5) Adjust the proportions of water, quartz sand, lime, and clay, determine the content of additives such as filtration reducer, and prepare cement-coated dense sandstone core samples with rock mechanical properties similar to those of natural dense sandstone cores and microcrack distribution patterns; adjust the proportions of water-quartz sand-lime-additives respectively, and configure the rock mechanical parameters of cement samples to be similar to those of dense sandstone; drill a through hole with a diameter of 25 mm in the center of the natural dense sandstone core; place the prepared cement slurry in regular hexahedral steel molds of different sizes, then place the natural dense sandstone core in the center and adjust the direction so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and mark it; then cast the steel wellbore in cement mortar and fill the wellbore perforations with urea in advance; cure at a constant temperature of 20°C for 28 days, then treat the surface of the large-size sample so that its parallelism and verticality meet the requirements, and then carry out the test.

[0055] (6) Using the principle of similarity, the test parameters for the fracturing construction of complex fracture networks in tight sandstone are prepared in the laboratory; based on the rock mechanical parameters, ground stress, brittleness index and microcrack distribution characteristics of tight sandstone, a large-scale true triaxial physical simulation test fracturing construction plan is designed, including fracturing fluid viscosity, fracturing fluid displacement, fracturing fluid type, fracturing fluid injection volume, etc.; tracer materials are added to the fracturing fluid to facilitate the analysis of the expansion law of complex fracture networks in tight sandstone.

[0056] (7) Carry out large-scale true triaxial physical simulation experiments on complex fracture networks in tight sandstone to test the expansion laws of complex fracture networks in tight sandstone under different construction parameters, different mineral components and contents, and different microcrack distributions; use the prepared large-scale tight sandstone rock samples and carry out large-scale true triaxial physical simulation experiments according to the set experimental plan.

[0057] (8) CT technology was used to scan and test large-scale rock samples and dense sandstone cores after the experiment, and a three-dimensional data body was established to analyze the changing characteristics of microcracks in dense sandstone cores. Different large-scale physical model experimental schemes were used to determine the influence of geological conditions, construction process parameters, mineral composition and microcrack distribution characteristics on the expansion of complex fracture networks in dense sandstone.

[0058] The specific steps include:

[0059] After testing, large-scale rock samples were taken and placed under industrial CT to analyze the distribution patterns of complex fractures in the large-scale rock samples. Natural dense sandstone cores were taken from the large-scale rock samples and scanned using CT to obtain three-dimensional natural fracture and fracture propagation data in the dense sandstone cores. The effects of dense sandstone microcracks and rock mineral composition on the propagation characteristics of complex fracture networks were analyzed.

[0060] The data volume is set according to the calibration position, and the change pattern of microcrack length, density and opening width before and after fracturing is obtained by analysis; the orientation of the complex fracture network is determined, and the relationship between the complex fracture network and the microcrack distribution is statistically analyzed to quantitatively evaluate the influence of microcrack parameters (length, density, opening degree, orientation) on the expansion of the complex fracture network; a relationship chart between the complex fracture network expansion parameters (length, orientation) and microcrack parameters is established to quantitatively characterize the expansion pattern of the complex fracture network.

[0061] The following are several specific embodiments of the present invention.

[0062] Example 1:

[0063] In a specific embodiment 1 of the present invention, Figure 1 FIG. 1 is a flow chart of a method for simulating and characterizing complex fracture network expansion in tight sandstone, which includes the following steps:

[0064] (1) Analysis and content determination of dense sandstone mineral components

[0065] Mineral component analysis showed that dense sandstone contains quartz, potassium feldspar, dolomite, clay minerals, etc., with proportions of 58%, 14%, 19%, and 9%, respectively. The brittle mineral content is 91% and the non-brittle mineral content is 9%. When preparing cement mortar, the mass ratio of cement sand to clay is adjusted to 9:1 to make their brittleness as similar as possible.

[0066] (2) Preparation of samples with rock mechanical properties similar to those of natural dense sandstone

[0067] The rock mechanical parameters of dense sandstone, such as elastic modulus, Poisson's ratio, and peak strength, were tested and analyzed. By adjusting the content and ratio of water-lime-quartz sand-clay-additives, the prepared rock samples were made similar to the rock mechanical properties of dense sandstone. The dense sandstone was taken from 3,300 m in the formation, and its elastic modulus, Poisson's ratio, and peak shear strength were found to be 29 GPa, 0.21, and 276 MPa, respectively. The lime used to prepare the artificial samples was PC52.5R composite silicate cement, 40-80 mesh quartz sand, montmorillonite, and a fluid loss reducer as the additive. The ratio of cement:quartz sand:water:clay was 4:4:1:1.

[0068] (3) Testing the magnitude and orientation of horizontal stress in natural dense sandstone

[0069] The horizontal stress of the tight sandstone formation is obtained by differential strain experiment, and the horizontal stress orientation of the formation is obtained by paleomagnetic method. The maximum horizontal stress is 62MPa, the minimum horizontal principal stress is 53MPa, and the orientation is NE36°. Figure 2 shown.

[0070] (4) CT scan test of natural dense sandstone core before experiment

[0071] The natural dense sandstone core is placed in an industrial CT scanning device to obtain the distribution parameters of the microcracks inside the dense sandstone, such as length, density, and orientation. The spatial coordinate system XYZ is established with the spatial origin (0, 0, 0) as the center point of the rock sample, and a three-dimensional data volume is established to obtain the distribution law of the microcracks, such as Figure 2 .

[0072] The distribution orientation of microcracks in natural tight sandstone cores was calibrated based on the orientation of maximum horizontal principal stress.

[0073] Attach Figure 3 As an example, the No. 1 crack in the CT microcrack image is identified based on the two pixel points (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 The distance formula between ) is:

[0074]

[0075] The microcrack length can be calculated by obtaining the ending coordinates of the microcrack in the 3D data volume. The corresponding starting and ending pixel coordinates of the crack are (3, 4, 5) and (6, 7, 8). The crack length is 5.196 mm. Assuming the microcrack orientation is α, then:

[0076]

[0077] The orientation of the microcrack can be calculated to be α = 45°.

[0078] Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal stress orientation is y = α-36 = 9°;

[0079] Draw a histogram of the length, density, azimuth angle α, and frequency of the deflection angle y of the maximum horizontal principal stress direction of the microcracks in the analysis section to determine the angle variation range and average value.

[0080] (5) Preparation of rock samples for large-scale physical simulation experiments of dense sandstone

[0081] Adjust the ratio of water-quartz sand-lime-additives, and configure the rock mechanical parameters of the cement sample to be similar to those of the dense sandstone rock mechanical parameters; drill a through hole with a diameter of 25mm in the center of the natural dense sandstone core; place the prepared cement slurry in a 300mmX300mmX300mm regular hexahedron steel mold, then place the natural dense sandstone core in the center and adjust the direction so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and mark it; then cast the steel wellbore in cement mortar together, and fill the wellbore perforations with urea in advance; cure at a constant temperature of 20℃ for 28 days, and then treat the surface of the large-size sample to make its parallelism and verticality meet the requirements, as shown in the attached figure. Figure 3 .

[0082] (6) Conducting large-scale true triaxial physical simulation experiments on complex fracture network expansion in tight sandstone

[0083] According to the experimental purpose and based on the principle of similarity, an experimental plan was formulated; tracer materials were added to the fracturing fluid, and experiments were carried out to test and analyze the fracture network expansion characteristics of tight sandstone layers.

[0084] (7) CT scanning analysis of large-size rock samples and dense cores

[0085] Using industrial CT, large-scale rock samples after the experiment were scanned to obtain three-dimensional data of the complex fracture network expansion morphology, and the expansion law of the complex fracture network was analyzed; the internal dense sandstone core was taken out and CT scanned to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the expansion law of the complex fracture network and the influence of microcracks were analyzed.

[0086] By comparing the scanning results of tight sandstone cores before and after the experiment, microcracks and complex fracture networks were identified.

[0087] The orientation of the complex fracture network after fracturing is calibrated using the previously calibrated maximum horizontal principal stress orientation; the relationship between the complex fracture network and the microcrack azimuth α is statistically analyzed to quantitatively evaluate the effect of the microcrack azimuth on fracture propagation; a frequency histogram of the complex fracture network length, density, azimuth α, and the deflection angle y of the maximum horizontal principal stress direction in the analysis section is drawn to determine the angle variation range and average value. By comparing the statistical results of the fracture deflection angle frequency histogram in step (4) and this step, the deflection angle range and the main deflection direction between the fracture and the maximum principal stress direction before and after fracturing can be obtained.

[0088] (8) By changing the triaxial stress and difference, fracturing operation parameters (fracturing fluid viscosity 1 mPa.s, displacement 15 ml / min, injection volume 200 ml), etc., the influence of the mineral composition of dense sandstone and microcracks on the expansion of complex fracture network was analyzed.

[0089] Example 2:

[0090] In the specific embodiment 2 of the present invention, as Figure 1 FIG. 1 is a flow chart of a method for simulating and characterizing complex fracture network expansion in tight sandstone, which includes the following steps:

[0091] (1) Analysis and content determination of dense sandstone mineral components

[0092] Mineral component analysis showed that dense sandstone contains quartz, potassium feldspar, dolomite, clay minerals, etc., with proportions of 68%, 10%, 12%, and 10%, respectively. The brittle mineral content is 90% and the non-brittle mineral content is 10%. When preparing cement mortar, the mass ratio of cement sand to clay is adjusted to 9:1 to make their brittleness as similar as possible.

[0093] (2) Preparation of samples with rock mechanical properties similar to those of natural dense sandstone

[0094] The rock mechanical parameters of dense sandstone, such as elastic modulus, Poisson's ratio, and peak strength, were tested and analyzed. By adjusting the content and ratio of water-lime-quartz sand-clay-additives, the prepared rock samples were made similar to the rock mechanical properties of dense sandstone. The dense sandstone was taken from 4500m in the formation, and its elastic modulus was analyzed to be 36GPa, Poisson's ratio was 0.20, and peak shear strength was 302MPa. The lime used to prepare the artificial sample was PC52.5R composite silicate cement, 20 / 40 mesh quartz sand, montmorillonite was used as clay, and a fluid loss reducer was used as additive. Among them, the cement:quartz sand:water:clay = 5:3:1:1.

[0095] (3) Testing the magnitude and orientation of horizontal stress in natural dense sandstone

[0096] The horizontal stress of the tight sandstone formation was obtained by differential strain experiment, and the horizontal stress orientation of the formation was obtained by paleomagnetic method. The maximum horizontal stress was 86MPa, the minimum horizontal principal stress was 72MPa, and the orientation was NE36°. Figure 2 shown.

[0097] (4) CT scan test of natural dense sandstone core before experiment

[0098] The natural dense sandstone core is placed in an industrial CT scanning device to obtain the distribution parameters of the microcracks inside the dense sandstone, such as length, density, and orientation. The spatial coordinate system XYZ is established with the spatial origin (0, 0, 0) as the center point of the rock sample, and a three-dimensional data volume is established to obtain the distribution law of the microcracks, such as Figure 2 .

[0099] The distribution orientation of microcracks in natural tight sandstone cores was calibrated based on the orientation of maximum horizontal principal stress.

[0100] Attach Figure 3 As an example, the No. 1 crack in the CT microcrack image is identified based on the two pixel points (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 The distance formula between ) is:

[0101]

[0102] The microcrack length can be calculated by obtaining the ending coordinates of the microcrack in the 3D data volume. The corresponding starting and ending pixel coordinates of the crack are (3, 4, 5) and (6, 7, 8). The crack length is 5.196 mm. Assuming the microcrack orientation is α, then:

[0103]

[0104] The orientation of the microcrack can be calculated to be α = 45°.

[0105] Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal stress orientation is y = α-36 = 9°;

[0106] Draw a histogram of the length, density, azimuth angle α, and frequency of the deflection angle y of the maximum horizontal principal stress direction of the microcracks in the analysis section to determine the angle variation range and average value.

[0107] (5) Preparation of rock samples for large-scale physical simulation experiments of dense sandstone

[0108] Adjust the ratio of water-quartz sand-lime-additives, and configure the rock mechanical parameters of the cement sample to be similar to those of the dense sandstone rock mechanical parameters; drill a through hole with a diameter of 25mm in the center of the natural dense sandstone core; place the prepared cement slurry in a 500mmX500mmX500mm regular hexahedron steel mold, then place the natural dense sandstone core in the center and adjust the direction so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and mark it; then cast the steel wellbore in cement mortar together, and fill the wellbore perforations with urea in advance; cure at a constant temperature of 20℃ for 28 days, and then treat the surface of the large-size sample to make its parallelism and verticality meet the requirements, as shown in the attached figure. Figure 3 .

[0109] (6) Conducting large-scale true triaxial physical simulation experiments on complex fracture network expansion in tight sandstone

[0110] According to the experimental purpose and based on the principle of similarity, an experimental plan was formulated; tracer materials were added to the fracturing fluid, and experiments were carried out to test and analyze the fracture network expansion characteristics of tight sandstone layers.

[0111] (7) CT scanning analysis of large-size rock samples and dense cores

[0112] Using industrial CT, large-scale rock samples after the experiment were scanned to obtain three-dimensional data of the complex fracture network expansion morphology, and the expansion law of the complex fracture network was analyzed; the internal dense sandstone core was taken out and CT scanned to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the expansion law of the complex fracture network and the influence of microcracks were analyzed.

[0113] By comparing the scanning results of tight sandstone cores before and after the experiment, microcracks and complex fracture networks were identified.

[0114] The orientation of the complex fracture network after fracturing is calibrated using the previously calibrated maximum horizontal principal stress orientation; the relationship between the complex fracture network and the microcrack azimuth α is statistically analyzed to quantitatively evaluate the effect of the microcrack azimuth on fracture propagation; a frequency histogram of the complex fracture network length, density, azimuth α, and the deflection angle y of the maximum horizontal principal stress direction in the analysis section is drawn to determine the angle variation range and average value. By comparing the statistical results of the fracture deflection angle frequency histogram in step (4) and this step, the deflection angle range and the main deflection direction between the fracture and the maximum principal stress direction before and after fracturing can be obtained.

[0115] (8) By changing the triaxial stress and difference, fracturing operation parameters (fracturing fluid viscosity 5mPa.s, displacement 25ml / min, injection volume 250ml), etc., the influence of the mineral composition of dense sandstone and microcracks on the expansion of complex fracture network was analyzed.

[0116] Example 3:

[0117] In the specific embodiment 3 of the present invention, Figure 1 FIG. 1 is a flow chart of a method for simulating and characterizing complex fracture network expansion in tight sandstone, which includes the following steps:

[0118] (1) Analysis and content determination of dense sandstone mineral components

[0119] Mineral component analysis showed that dense sandstone contains quartz, potassium feldspar, dolomite, clay minerals, etc., with proportions of 74%, 12%, 6%, and 8%, respectively. The brittle mineral content is 92% and the non-brittle mineral content is 8%. When preparing cement mortar, the mass ratio of cement sand to clay is adjusted to 9:1 to make their brittleness as similar as possible.

[0120] (2) Preparation of samples with rock mechanical properties similar to those of natural dense sandstone

[0121] The rock mechanical parameters of dense sandstone, such as elastic modulus, Poisson's ratio, and peak strength, were tested and analyzed. By adjusting the content and ratio of water-lime-quartz sand-clay-additives, the prepared rock samples were made similar to the rock mechanical properties of dense sandstone. The dense sandstone was taken from 2000m in the formation, and its elastic modulus was analyzed to be 23GPa, Poisson's ratio was 0.22, and peak shear strength was 216MPa. The lime used to prepare the artificial sample was PC52.5R composite silicate cement, 40 / 60 mesh quartz sand, montmorillonite was used as clay, and a fluid loss reducer was used as additive. Among them, the cement:quartz sand:water:clay = 3:5:1:1.

[0122] (3) Testing the magnitude and orientation of horizontal stress in natural dense sandstone

[0123] The horizontal stress of the tight sandstone formation is obtained by differential strain experiment, and the horizontal stress orientation of the formation is obtained by paleomagnetic method. The maximum horizontal stress is 42MPa, the minimum horizontal principal stress is 31MPa, and the orientation is NE36°. Figure 2 shown.

[0124] (4) CT scan test of natural dense sandstone core before experiment

[0125] The natural dense sandstone core is placed in an industrial CT scanning device to obtain the distribution parameters of the microcracks inside the dense sandstone, such as length, density, and orientation. The spatial coordinate system XYZ is established with the spatial origin (0, 0, 0) as the center point of the rock sample, and a three-dimensional data volume is established to obtain the distribution law of the microcracks, such as Figure 2 .

[0126] The distribution orientation of microcracks in natural tight sandstone cores was calibrated based on the orientation of maximum horizontal principal stress.

[0127] Attach Figure 3As an example, the No. 1 crack in the CT microcrack image is identified based on the two pixel points (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 The distance formula between ) is:

[0128]

[0129] The microcrack length can be calculated by obtaining the ending coordinates of the microcrack in the 3D data volume. The corresponding starting and ending pixel coordinates of the crack are (3, 4, 5) and (6, 7, 8). The crack length is 5.196 mm. Assuming the microcrack orientation is α, then:

[0130]

[0131] The orientation of the microcrack can be calculated to be α = 45°.

[0132] Combined with the determined maximum horizontal principal stress orientation, the deflection angle between the microcrack and the maximum horizontal stress orientation is y = α-36 = 9°;

[0133] Draw a histogram of the length, density, azimuth angle α, and frequency of the deflection angle y of the maximum horizontal principal stress direction of the microcracks in the analysis section to determine the angle variation range and average value.

[0134] (5) Preparation of rock samples for large-scale physical simulation experiments of dense sandstone

[0135] Adjust the ratio of water-quartz sand-lime-additives, and configure the rock mechanical parameters of the cement sample to be similar to those of the dense sandstone rock mechanical parameters; drill a through hole with a diameter of 25mm in the center of the natural dense sandstone core; place the prepared cement slurry in a 200mmX200mmX200mm regular hexahedron steel mold, then place the natural dense sandstone core in the center and adjust the direction so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and mark it; then cast the steel wellbore in cement mortar together, and fill the wellbore perforations with urea in advance; cure at a constant temperature of 20℃ for 28 days, and then treat the surface of the large-size sample to make its parallelism and verticality meet the requirements, as shown in the attached figure. Figure 3 .

[0136] (6) Conducting large-scale true triaxial physical simulation experiments on complex fracture network expansion in tight sandstone

[0137] According to the experimental purpose and based on the principle of similarity, an experimental plan was formulated; tracer materials were added to the fracturing fluid, and experiments were carried out to test and analyze the fracture network expansion characteristics of tight sandstone layers.

[0138] (7) CT scanning analysis of large-size rock samples and dense cores

[0139] Using industrial CT, large-scale rock samples after the experiment were scanned to obtain three-dimensional data of the complex fracture network expansion morphology, and the expansion law of the complex fracture network was analyzed; the internal dense sandstone core was taken out and CT scanned to obtain a three-dimensional data volume containing microcracks and complex fracture networks, and the expansion law of the complex fracture network and the influence of microcracks were analyzed.

[0140] By comparing the scanning results of tight sandstone cores before and after the experiment, microcracks and complex fracture networks were identified.

[0141] The orientation of the complex fracture network after fracturing is calibrated using the previously calibrated maximum horizontal principal stress orientation; the relationship between the complex fracture network and the microcrack azimuth α is statistically analyzed to quantitatively evaluate the effect of the microcrack azimuth on fracture propagation; a frequency histogram of the complex fracture network length, density, azimuth α, and the deflection angle y of the maximum horizontal principal stress direction in the analysis section is drawn to determine the angle variation range and average value. By comparing the statistical results of the fracture deflection angle frequency histogram in step (4) and this step, the deflection angle range and the main deflection direction between the fracture and the maximum principal stress direction before and after fracturing can be obtained.

[0142] (8) By changing the triaxial stress and difference, fracturing operation parameters (fracturing fluid viscosity 10 mPa.s, displacement 35 ml / min, injection volume 300 ml), etc., the influence of the mineral composition of dense sandstone and microcracks on the expansion of complex fracture network was analyzed.

[0143] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art may modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features therein. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

[0144] Except for the technical features described in the specification, all other technical features are known technologies to those skilled in the art.

Claims

1. A method for simulating and characterizing complex fracture network expansion in tight sandstone, characterized by: The simulation and characterization method for complex fracture network expansion in tight sandstone includes: Step 1, analyzing the mineral composition of the tight sandstone core; Step 2: Testing rock mechanical parameters of tight sandstone core; Step 3: testing the maximum horizontal principal stress and orientation of the tight sandstone core; Step 4: Use CT technology to scan the distribution pattern of microcracks in the dense sandstone core; Step 5: preparing a cement-coated dense sandstone core sample having similar rock mechanical properties and microcrack distribution patterns to those of natural dense sandstone cores; Step 6: Develop test parameters for fracturing complex fracture networks in tight sandstone in the laboratory based on the similarity principle. Step 7: Conduct a large-scale true triaxial complex fracture network physical simulation experiment to explore the expansion law of complex fracture networks in tight sandstone under different microcrack distributions; Step 8: Use CT technology to create a three-dimensional data volume of the core before and after the experiment to represent the complex fracture network.

2. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 1, a mineral component analyzer is used to analyze the mineral composition and content of the dense sandstone, including quartz, calcite, and clay, and the dense sandstone brittleness index is calculated to determine the complexity of the fracture network.

3. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 2, the rock mechanics parameters of the dense sandstone core, such as shear strength, elastic modulus, and Poisson's ratio, are tested; and the rock mechanics parameters, such as peak strength, elastic modulus, and Poisson's ratio, of the dense sandstone core are tested using a triaxial testing device for rock mechanics parameters.

4. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 3, the maximum and minimum horizontal principal stresses of the core are tested using differential strain experiments, the original orientation of the tight sandstone core is determined using paleomagnetic tests, and a tight sandstone core coordinate system is established; the in-situ stress orientation is determined based on the tight sandstone core logging data.

5. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 4, characterized in that: In step 3, the logging data includes sonic, caliper, and resistivity logging.

6. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 4, CT technology is used to scan the distribution of microcracks in the dense sandstone core before the experiment to obtain its distribution pattern, including microcrack density, length, and openness. A digital image space coordinate system is established to obtain a three-dimensional data volume to calculate the azimuth and inclination of the microcracks. The image is processed to make it clearer and the microcracks more obvious. Then, weighted average interpolation is performed on the image, and a standard layer is selected and set as the initial image.

7. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 6, characterized in that: In step 4, the microcrack parameters of tight sandstone are obtained by taking the center point of the rock sample as the coordinate origin (0,0,0) of the spatial coordinate system XYZ; the three-dimensional data volume of the core is obtained by CT scanning, and the coordinates of the first and last two pixel points of the microcrack (X i , Y i , Z i ), (X i+1 , Y i+1 , Z i+1 ) to determine the length and distribution azimuth angle α of the microcracks; draw a frequency histogram of the azimuth angle α of the microcracks in the analysis section and the deflection angle Y in the direction of the maximum horizontal principal stress to determine the range of variation of the microcrack parameters.

8. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 7, characterized in that: In step 4, the deflection angle Y between the microcracks in the tight sandstone and the maximum horizontal stress orientation is determined based on the calibrated maximum horizontal stress orientation of the tight sandstone.

9. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 5, the proportions of water, quartz sand, lime, and clay are adjusted, the content of the fluid loss reducer additive is determined, and a cement-coated dense sandstone core sample having rock mechanical properties and microcrack distribution similar to those of a natural dense sandstone core is prepared.

10. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 9, characterized in that: In step 5, the proportions of water, quartz sand, lime and additives are adjusted respectively to prepare the rock mechanical parameters of the cement sample similar to those of the dense sandstone rock mechanical parameters; a through hole with a diameter of 25 mm is drilled in the center of the natural dense sandstone core; the prepared cement slurry is placed in regular hexahedral steel molds of different sizes, and then the natural dense sandstone core is placed in the center and the direction is adjusted so that the maximum horizontal principal stress orientation of the dense sandstone core is parallel to the side, and marked; then the steel wellbore is cast in the cement mortar together, and the wellbore perforations are filled with urea in advance; it is cured at a constant temperature of 20°C for 28 days, and then the surface of the large-size sample is treated to make its parallelism and verticality meet the requirements, and then the test can be carried out.

11. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 6, the similarity principle is used to prepare the laboratory test parameters for the fracturing construction of complex fracture networks in tight sandstone. Based on the rock mechanical parameters, ground stress, brittleness index and microcrack distribution characteristics of tight sandstone, a large-scale true triaxial physical simulation test fracturing construction plan is designed, including fracturing fluid viscosity, fracturing fluid displacement, fracturing fluid type, and fracturing fluid injection volume. Tracer materials are added to the fracturing fluid to facilitate the analysis of the expansion law of complex fracture networks in tight sandstone.

12. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 7, a large-scale true triaxial physical simulation experiment of complex fracture network in tight sandstone is carried out to test the expansion law of complex fracture network in tight sandstone under different construction parameters, different mineral components and contents, and different microcrack distributions; using the prepared large-scale tight sandstone rock samples, a large-scale true triaxial physical simulation experiment is carried out according to the set experimental plan.

13. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 1, characterized in that: In step 8, CT technology is used to scan and test large-scale rock samples and dense sandstone cores after the experiment, establish a three-dimensional data body, and analyze the changing characteristics of microcracks in the dense sandstone cores. Different large-scale physical model experimental schemes are used to determine the influence of geological conditions, construction process parameters, mineral composition, and microcrack distribution characteristics on the expansion of complex fracture networks in dense sandstone.

14. The method for simulating and characterizing complex fracture network expansion in tight sandstone according to claim 13, wherein: Step 8 specifically includes: After testing, large-scale rock samples were taken and placed under industrial CT to analyze the distribution patterns of complex fractures in the large-scale rock samples. Natural dense sandstone cores were taken from the large-scale rock samples and scanned using CT to obtain three-dimensional natural fracture and fracture propagation data in the dense sandstone cores. The effects of dense sandstone microcracks and rock mineral composition on the propagation characteristics of complex fracture networks were analyzed. The data volume is set according to the calibration position, and the change pattern of microcrack length, density and opening width before and after fracturing is obtained by analysis; the orientation of the complex fracture network is determined, and the relationship between the complex fracture network and the microcrack distribution is statistically analyzed, and the influence of microcrack parameters including length, density, opening degree and orientation on the expansion of the complex fracture network is quantitatively evaluated; a relationship chart of complex fracture network expansion parameters including length, orientation and microcrack parameters is established to quantitatively characterize the expansion pattern of the complex fracture network.

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